Doubly [SiMe2]-bridged metallocenes (1,2-SiMe2)(2){eta(5)-C5H2-4-R}{eta(5)-C5H-3,5-(CHMe2)(2)}ZrCl2 (R = H (1a), CHMe2 (1b), SiMe3 (1c), CHMe(CMe3) (1d), (+)-menthyl (1e)), when activated by methylaluminoxane (MAO), catalyze propylene polymerization with high activities. The preparations and X-ray structures of the dilithio salt of an enantiopure, doubly silylene-bridged bis(cyclopentadienyl) ligand, Li-2[(1,2-Me2Si)(2)-{C5H2-4-(1R,2S,5R-menthyl)}{(C5H-3,5-(CH as well as the corresponding zirconocene dichloride, [(1,2-Me2Si)(2){eta(5)-C5H2-4-( 1R,2S,5R-menthyl)}{eta(5)-C5H-3,5-(CHMe2)(2)}]ZrCl2 (1e), are reported. The C-s,symmetric systems la-e are highly regiospecific and syndiospecific (>99.5%) in neat propylene. At lower propylene concentrations, polymers with lower molecular weights and tacticity (mostly m-type stereoerrors) are obtained. The microstructures of polymers produced under differing reaction conditions are consistent with stereocontrol dominated by a site epimerization process, an inversion of configuration at zirconium resulting from the polymer chain swinging from one side of the metallocene wedge to the other without monomer insertion. The relative importance of chain epimerization (at the beta carbon) has been established by parallel polymerization of 2-d(1)-propylene and d(0)-propylene with 1b/MAO at low propylene concentrations. The C-1-symmetric systems 1d,e/MAO display an unusual dependence of stereospecificity on propylene concentration, switching from isospecific to syndiospecific with increasing propylene pressure, consistent with a competitive unimolecular site epimerization process and a bimolecular chain propagation. The microstructures of the polypropylenes produced by 1d/MAO and 1e/MAO with [r] approximate to 50% resemble the hemiisotactic microstructure produced by Me2C(eta(5)-C5H3-3-Me)(eta(5)-C13H8)ZrCl2 (2b)/MAO. Contrastingly, the hemiisotactic polypropylene microstructure obtained with 2b/MAO is found to be maintained at all propylene concentrations examined.